This application is a Continuation of International Application PCT/FI00/00413 filed on May 9, 2000 now is WO 00/76622 A1 published Dec. 21, 2000, which designated the U.S. and was published under PCT Article 21(2) in English.
The invention relates to filter cloth composed of a plurality of yarns in the transverse and the longitudinal directions, the filter cloth comprising a filtering portion having a structure and density according to desired filtering characteristics for separating liquid from a mixture consisting of solids and liquid, and which filter cloth is further to be arranged against a filtering element in a filtering apparatus, and the structure of the underside of the filter cloth, i.e. a portion facing the filtering element, comprises thicker yarns than a portion facing the upper side of the cloth for the flow of liquid filtered by the cloth.
The invention further relates to a filtering module to be arranged on a filtering element as a filtering surface when liquid is separated from a mixture consisting of solids and liquid by means of a filtering apparatus, which filtering module is made of filter cloth comprising a filtering layer composed of yarns in the transverse and the longitudinal directions, thicker yarns than the rest of the yarns of the cloth having been arranged while manufacturing the cloth in the bottom of the filter cloth for the flow of liquid.
Different machine screens, such as disc filters and drum filters, are used for removing liquid from a mixture consisting of liquid and dry solids. A disc filter typically comprises a plurality of triangular filter segments arranged in a rotating frame onto which segments are arranged filter bags made of filter cloth. The liquid in the mixture to be filtered is allowed to penetrate through the filter cloth and further into a segment through openings extending to the surface of the filter segment. In addition to the segments, depending on the structure of the filter, different box-like and cylinder-like pieces and other elements are used onto which a bag or a similar filtering module made of filter cloth is stretched by means of different mechanical tightening parts or, nowadays increasingly often, by thermal contraction. It has been found out that if compact filter cloth is in use allowed to be pressed tightly against the surface of the filtering element, the flow of the passed liquid is hindered between the filtering cloth and the element to the openings of the element. In order to prevent this problem, a separate xe2x80x9cadditional wire bagxe2x80x9d has been introduced between the actual filtering bag and the filtering element. The backing wire bag is made of a netlike texture forming a loose, highly permeable layer in the transverse direction of the surface of the cloth between the compact filtering bag and the surface of the element. Although such a separate backing wire bag does work relatively well in practice, fitting the backing wire bag is, however, an additional stage of operation. Furthermore, the manufacturing also causes additional costs.
SE 431 826 discloses filter cloth for separating solid particles and liquid, comprising at least two interwoven layers: a filtering layer and a supporting layer. In the supporting layer, i.e. in the portion facing the filtering element, thicker yarns are to be used in order to achieve a more coarse bottom. A drawback of the disclosed solution is, however, that the liquid that has already passed the filtering layer in the direction of the surface of the filtering element flows inside the supporting texture of the filter cloth. As normally, the texture comprises yarns travelling crosswise and having different directions, whereby the yarns form obstacles to an efficient flow of liquid.
An object of the present invention is to provide filter cloth and a filtering module manufactured from the same which avoid drawbacks of the known solutions.
The filter cloth of the invention is characterized in that the underside of the filter cloth comprises substantially parallel yarns that are thicker than the rest of the yarns of the cloth, and that the thicker yarns form parallel channels therebetween in order to enable the filtered liquid to flow in the direction of the surface of the filtering element between the filtering portion of the filter cloth and the filtering element.
The filtering module of the invention is further characterized in that the filtering module is manufactured from filter cloth whose underside, i.e. the surface to be against the filtering element, comprises substantially parallel yarns that are thicker than the rest of the yarns of the cloth, and that channels are formed between the thicker yarns, in which channels the liquid filtered by the cloth is allowed to flow in the direction of the surface of the filtering element.
The idea underlying the invention is that the filter cloth is produced such that while weaving the cloth, parallel, thicker yarns are arranged on the underside of the cloth, i.e. the surface facing the filtering element, the yarns forming therebetween parallel channels on the underside of the cloth to enable the filtered liquid to flow in the direction of the surface of the filtering element. The parallel, thicker yarns are additional yarns in regard to the rest of the structure of the filter cloth; hence, they are irrelevant to the rest of the structure of the cloth. Furthermore, the yarns used in the cloth that travel at the thicker yarns and have the same direction as the thicker yarns are preferably multifilaments, whereby they set tightly in the cloth in the vicinity of the monofilament, thicker yarns. Hence, the thicker yarns do not impair the density of the cloth. Furthermore, the filter cloth can be directed in the filtering module such that the flow channels on the underside of the cloth are located according to the positions of the openings, i.e. the channels direct the liquid to the openings. Owing to the thick yarns, the dense, filtering texture portion of the filtering module is located at a distance from the surface of the filtering element.
An advantage of the invention is that by manufacturing filter bags to be fitted onto different filtering elements and similar replaceable filtering modules from the filter cloth of the invention, the use of separate backing wire bags is avoided. Hence, it is easier and quicker to change the filter bags, and no further costs result from an additional bag. Furthermore, even thin filter cloth can be made stiff thanks to the thick yarns in the bottom of the cloth. A stiff filtering module stays better in place and thus lasts longer in use. Furthermore, the thick yarns in the bottom protect the actual filtering layer of the cloth, whereby sharp edges or the like of the filtering element cannot damage the cloth. In regard to the SE publication mentioned in the description of the prior art, the filter cloth of the invention can be more readily cleaned during use since dirt does not become clogged up in the open channels in the bottom of the cloth of the invention in such a manner as dirt can be clogged in different texture structures. A substantial advantage is that now the filtered liquid does not flow inside the texture structure but in the flow channels formed for the purpose, wherein there are no yarns in the transverse direction of the channels to block the flow. Furthermore, the flow of liquid can be directed in a desired manner when the filtering module is manufactured such that the parallel flow channels in the bottom of the filter cloth are arranged according to the positions of the openings and the operation of the filtering element. It is then possible to arrange the openings of the filtering element and the channels of the filter cloth in such a manner that liquid is allowed to enter the openings of the filtering element and further on as efficiently as possible. All in all, by using the filter cloth of the invention, the working costs of the filtering apparatus can be reduced and the filtering efficiency improved.